This invention relates to shaping heat softened sheet material, and in particular to shaping heat softened glass sheets between an upper mold and a lower combination flexible/rigid ring mold.
Shaped and tempered glass sheets are widely used as windows in vehicles such as automobiles and the like. To fabricate these windows, flat glass sheets must be shaped to precisely defined curvatures dictated by the shape and outline of the window frame openings in the vehicle. It is important that the windows meet stringent optical requirements and be free of optical defects that would tend to interfere with clear viewing through the window.
Commercial production of shaped glass sheets commonly includes the steps of serially conveying the glass sheets through a tunnel type furnace where they are heated to their heat deformation temperature and thereafter conveying the heat softened sheets into a shaping station where they are shaped by pressing each sheet between a pair of vertically, aligned upper and lower shaping molds. After shaping, the molds separate with the shaped glass sheet remaining secured to the upper mold by vacuum. A transfer ring having an outline and shape conforming to the desired curvature of the glass sheet slightly inboard of its perimeter, moves beneath the upper mold which thereafter releases the vacuum and deposits the shaped glass sheet on the ring. The ring then transfers the shaped glass sheet into a cooling station for controlled cooling.
The lower mold in such sheet shaping arrangement may include a rigid shaping ring as disclosed in U.S. Pat. No. 4,496,386 to Hymore et al. or a flexible shaping ring as disclosed in U.S. Pat. No. 4,830,650 to Kelly. During shaping, the lower mold moves upward from a position below the conveying surface of the convey rolls to lift the glass sheet off the conveying rolls and into engagement with the upper mold. Each of these shaping arrangements have certain shortcomings. When using a rigid ring, since the ring has an elevational configuration generally corresponding to the final desired peripheral shape of the glass sheet, the ring does not simultaneously contact the entire marginal edge of the glass sheet as it initially lifts the sheet off the conveyor rolls. Rather, the rigid ring progressively engages the marginal edge with the higher points of the ring contacting the ring first. As a result, the glass may slide along the sheet engaging surface of the ring as the glass is shaped. When using a flexible ring mold, the ring has a flat undeformed configuration when it initially engages the glass sheet so that the entire marginal edge of the sheet is contacted simultaneously by the ring as the ring lifts the sheet off the conveyor rolls. However, as the glass is pressed against the upper mold, the pressure applied by the flexible ring depends on the member used to maintain the ring in an undeformed configuration. For example, in a ring as disclosed in U.S. Pat. No. 4,830,650, the pressing force depends on the spring constants of the springs which support the flexible ring.
It would be advantageous to provide an arrangement whereby the lower mold simultaneously engages the entire marginal edge portion of a glass sheet to be shaped, preliminarily shapes the glass sheet and further positively presses the marginal edge against an upper shaping mold to ensure that the configuration of the marginal edge portion of the sheet fully conforms to the shape of the upper mold.